The recent progress of digital and highly precise optical apparatus and instruments has brought about a rapid increase in a demand for lenses and other optical elements for digital and video cameras and, among others, an ever-increasing demand for highly refractive and highly dispersive glass as desired from an optical engineering standpoint. Highly refractive and highly dispersive optical glass having a refractive index of 1.8 or above and an Abbe number of 30 or less is disclosed in a number of documents including the following:
(1) Patent Document 1—JP-A-S60-5037
(2) Patent Document 2—JP-A-S59-8637
(3) Patent Document 3—JP-A-2001-342035
(4) Patent Document 4—JP-A-2004-155639
Patent Document 1 discloses SiO2—TiO2—Nb2O5—BaO—PbO—Na2O type optical glass containing PbO as its essential constituent and having high refractivity and high dispersion. Patent Document 2 discloses SiO2—P2O5—TiO2—BaO—R2O type optical glass not containing PbO, but having high refractivity and high dispersion. Patent Document 3 discloses SiO2—TiO2—Nb2O5—BaO—Na2O type optical glass not containing PbO, As2O3 or F, but containing a large amount of Nb2O5, and having high refractivity and high dispersion. Patent Document 4 discloses SiO2—TiO2—Nb2O5—BaO—Na2O type optical glass not containing PbO, As2O3 or F, but containing a large amount of TiO2, and having high refractivity and high dispersion.
However, the optical glass disclosed in Patent Document 1 has an environmental problem because of its PbO. The optical glass disclosed in Patent Document 2 has its high refractivity and high dispersion difficult to maintain because of its low content of a material imparting high refractivity, such as Nb2O5.
The optical glass disclosed in Patent Document 3 or 4 is defective in stability and resistance to devitrification at the time of press working, though its high content of a material imparting high refractivity, such as TiO2 or Nb2O5 make it relatively easy to produce glass of high refractivity and high dispersion.
As it is generally true that while the coloring resistance of glass of high refractivity and dispersion is lowered by TiO2 or Nb2O5 which it contains for improved refractivity, a reduction of the proportion of TiO2 or Nb2O5 for the improved coloring resistance of glass makes it difficult to maintain its high refractivity and dispersion, it has been very difficult to obtain those two features at the same time.
Another drawback arising from a high proportion of TiO2 or Nb2O5 is the devitrification of glass which occurs easily at the time of its reheating and press working, depending on its composition. When an optical element is formed from a glass ingot prepared by melting optical glass, shaping it into a sheet of glass and cutting or otherwise cold working it, its reheating and press working are required, and while glass composed mainly of SiO2 relies on a high proportion of TiO2 or Nb2O5 for improved refractivity, it is known that SiO2 glass containing a high proportion of TiO2 or Nb2O5 undergoes devitrification easily when heated to, say, 800° C.
Accordingly, there has been a strong demand for a glass composition which is high in transparency and does not undergo devitrification by any material added to it to give it high refractivity to make an optical element of high refractivity and dispersion.